Add API for building attribute getter free functions. #2684

Job	Run time
VFX2019 gcc6/C++14 llvm9 py2.7 boost1.66 exr2.3 oiio2.3 sse2	14m 16s
VFX2019 clang9/C++14 llvm9 oiio-release boost1.66 avx2 exr2.3 py2.7	9m 12s
VFX2019 GPU Cuda10 gcc6/C++14 llvm10 py2.7 boost-1.70 exr-2.3 OIIO-master avx2	3m 48s
VFX2020 gcc6/C++14 llvm10 py3.7 boost1.70 exr2.4 oiio2.2 sse4	12m 53s
VFX2021 gcc9/C++17 llvm11 py3.7 boost1.73 exr2.5 oiio2.3 avx2 batch-b8avx2	31m 23s
VFX2022 gcc9/C++17 llvm13 py3.9 boost1.76 exr3.1 oiio-rel avx2	30m 39s
VFX2022 clang12/C++17 llvm12 oiio-master boost1.73 exr3.1 py3.9 avx2 batch-avx512	39m 30s
VFX2022 icc/C++17 llvm14 py3.9 boost1.76 exr3.1 oiio-master avx2	50m 15s
VFX2022 icx/C++17 llvm14 py3.9 boost1.76 exr3.1 oiio2.3 avx2	41m 44s
VFX2023 gcc11/C++17 llvm15 py3.10 boost1.80 exr3.1 oiio-rel avx2	36m 58s
VFX2019 oldest everything gcc6/C++14 llvm9 py2.7 boost1.66 oiio2.3 no-simd exr2.3	8m 43s
VFX2022 sanitizers	35m 2s
VFX2022 clang-format	4m 1s
macos-11 appleclang13/C++17 py3.10 MacOS-11	39m 30s
macos-12 appleclang13/C++17 py3.10 MacOS-12	21m 53s
macos-13 appleclang14/C++17 py3.11 MacOS-13	15m 37s
Ubuntu Debug gcc7/C++14 llvm9 py2.7 oiio2.3 exr2.4 sse4 boost1.65 exr2.4	40m 5s
Ubuntu gcc10/C++17 llvm10 oiio-release boost1.65 exr2.5 avx2	13m 25s
Ubuntu latest releases gcc11/C++17 llvm13 boost1.71 exr3.1 py3.9 avx2 batch-b16avx512	26m 48s
Ubuntu bleeding edge gcc12/C++17 llvm14 oiio/ocio/exr/pybind-master boost1.71 py3.10 avx2 batch-b16avx512	36m 12s
Ubuntu clang14/C++17 llvm14 boost1.71 exr3.1 py3.8 avx2 batch-b16avx512	37m 18s
	9h 9m 12s

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